1. Field of the Invention
The present invention relates to an image creating method and an image creating system.
2. Description of the Related Art
In Japanese Patent Application Laying-Open Publication No. 2003-158633, there is disclosed a technique for image creation using a dithering method to halftone a set of grayscale or multi-tone data of an original image frame.
The dithering method will be briefly described with reference to FIGS. 4 to 7.
Now assumed is a set of original image data (i.e., a set of data on pixels in an original image frame) all being a data of 8 bits, i.e., 256 tones from “0 (white)” to “255 (black)”.
Given in FIG. 4 are diagrams of exemplary dither arrays of a halftone cell in a digital halftone processing for an output under conditions: resolution 300 dpi, screen ruling frequency 100 lpi, and screen angle 0 deg. Generally, the “number of reproducible different tones” can be represented such that (output resolution/ruling frequency)2+1. Now, (300/100)2+1=10 is the reproducible tone number, so the dither arrays have a size of 10−1=9=3×3 grid sections. Such sections one-to-one correspond to pixels (duotone dots) in the halftone cell, and will be referred herein simply to “pixels”. FIG. 4A gives an exemplary tone distribution in which ranks “1” to “9” of reproducible tones are distributed to 3×3 arrayed pixels for each cell, in an order starting from a central pixel (emphasis added in the figure). For a halftone representation of 256 tones of image data, actually employed is a dither array of thresholds that correspond, as in FIG. 4B, to the tone ranks in the array of FIG. 4A, as they are multiplied by “255”/(3×3+1)=“25.5” and rounded.
In the original image frame, each pixel has a tone within a range of levels “0” to “255”, and pixels residing in a local image region corresponding to a halftone cell have a concentration commensurate with an average of their tones, i.e., a grayscale value. On the other hand, in the halftone cell, each pixel has simply a tone of either level “1” or “0” corresponding to a black dot or a white dot (as a dot-less white base), respectively, and does not have any intermediate tone in between. Instead, as illustrated in FIG. 5 for 3×3 dither arrays, the halftone cell provides a grayscale illusion within a grayscale interval depending on the pattern of distribution of thresholds, or proportion of areas of black dots.
FIG. 6 illustrates an exemplary process using 3×3 dither arrays to dither an original gray solid image frame of a grayscale value “80”. This example reads a set of data on 3×3 pixels in a current local region of the original image frame, and compares the grayscale value of this region with grayscale intervals of 3×3 dither arrays in FIG. 5, to determine or identify a dither array of a corresponding threshold distribution pattern to be employed to output a cell of halftone image of a corresponding grayscale value. In this halftone cell, each pixel for which the dither array has set a threshold equivalent to or smaller than the grayscale value constitutes a black dot, and the remaining pixels appear as white dots. After that, a set of data on 3×3 pixels in a subsequent local region is read, and likewise processed. This corresponds to shifting a dither array from the current region to the subsequent region. Like this, an entire region of the original image frame is scanned, and halftone processed.
In such the dithering method, the grayscale intervals in halftone can be refined by changing the dither array to an expanded size, e.g. from 3×3 arrays in FIGS. 4A and 4B to 6×6 arrays in FIGS. 7A and 7B, so far as the reproducible tone number permits. For example, the 6×6 arrays permit the scale of 256 tones in original image to be parted into 62+1=37 intervals by halftone, allowing for a refined grayscale representation in comparison with the 3×3 arrays.